The overall goal of this revised application is to investigate mechanisms by which airway progenitor cells acquire and balance their different cell phenotypes during development and during regeneration in adult life. More specifically, we will focus on the role of Notch and its regulation by Ascl1 in the selection of cell fates in developing airways and when airways are repopulated after injury. We have recently reported that Notch signaling in the embryonic airway epithelium is essential for establishing the balance of secretory and non-secretory cell fates. Disruption of Notch signaling prevents Clara cells from forming and results in airways overpopulated by neuroendocrine and ciliated cells. How does Notch influence cell fate selection? Which ligands are critical for this process and do they drive any specific lineage program in the airways? Studies in adult mice injured by Naphthalene suggest that a population of Clara cells resistant to this compound is able to give rise to a balanced proportion of secretory and ciliated cells. How this is achieved is currently unknown. We have preliminary evidence that Notch may regulate this process. Here we propose to address these issues in three specific aims. Aim 1: Study mechanisms that balance epithelial cell phenotypes looking at the role of different Notch ligands in mouse genetic models in which Jagged or Delta ligands were inactivated; Aim 2: Characterize lineage and fate of Ascl1-expressing cells and their influence in the neighbor airway epithelium using in vivo and airway epithelial culture models, and, Aim 3: Investigate mechanisms of Notch-mediated cell fate choice in regenerating airways using a Naphthalene model of injury repair and pharmacologic and genetic approaches to inactivate Notch.